1. Field of the Invention
This invention relates to a new plastisol composition based on styrene copolymers, plasticizers and inorganic fillers and, optionally, other standard additives.
Plastisols are generally understood to be dispersions of organic polymers in plasticizers which gel on heating to relatively high temperatures and harden on cooling. By far the majority of plastisols still commonly used in practice today contain finely powdered polyvinyl chloride (PVC) which is dispersed in a liquid plasticizer and forms a paste. Corresponding polyvinyl chloride plastisols are used for various applications. They are employed inter alia as sealing compounds, for example for seam sealing in metal containers or as flanged seam adhesives in the metal industry, as corrosion-inhibiting coatings for metals (for example as an underseal for motor vehicles), for impregnating and coating substrates of textile materials (for example as a coating for carpet backings), as cable insulations, etc.
Unfortunately, numerous problems are involved in the production and application of PVC plastisols. Even the production of PVC itself is not without problems because the health of personnel working in the production plants is endangered by the monomeric vinyl chloride. In addition, residues of monomeric vinyl chloride in PVC can also be a health hazard during subsequent processing or to end users although, in general, the contents are only in the ppb range.
A particularly serious problem in the application of PVC plastisols is that PVC is both heat-sensitive and light-sensitive and tends to eliminate hydrogen chloride. This is a serious problem in particular when the plastisol has to be heated to a relatively high temperature because the hydrogen chloride released under these conditions has a corrosive effect and attacks metal substrates. This applies in particular when relatively high baking temperatures are applied to shorten the gel time or when locally high temperatures occur, as in spot welding.
The biggest problem arises in the disposal of PVC-containing waste. In addition to hydrogen chloride, dioxins which are known to be highly toxic can be formed. In conjunction with steel scrap, PVC residues can lead to an increase in the chloride content of the steel melt which is also a disadvantage.
Accordingly, the problem addressed by the present invention was to provide a plastisol composition free from polyvinyl chloride which would be equivalent to PVC plastisols in its performance properties.
Polyurethane- or acrylate-based coating compositions are already known and are used instead of PVC plastisols, for example in the automotive industry. Two-component polyurethane systems differ basically from standard plastisols in terms of application; the complicated equipment required for their processing is generally not available to the user.
One-component polyurethane systems are also known but are all attended by a number of other disadvantages, namely:
Moisture-curing systems have a high viscosity and, accordingly, cannot be applied without solvents. PA1 In the case of systems containing blocked isocyanate groups, the volatility of the blocking agent can lead to bubble formation in thick layers, in addition to which the application temperature range of 150.degree. C. to 180.degree. C. often cannot be adhered to for the baking conditions. PA1 Aqueous PU dispersions do not fit into the usual production sequence on account of the evaporating water. PA1 Microencapsulated polyurethane systems lack shear stability which leads to gelation in the pumps during application. PA1 a) styrene and/or .alpha.-methyl styrene and/or p-methyl styrene and PA1 b) 3 to 20% by weight (based on the copolymer) of methacrylic acid and/or acrylic acid and/or itaconic acid. PA1 Thermal stability and high-temperature dimensional stability are considerably improved by crosslinking. PA1 The soft segments of the additives provide for flexibilization and greater elasticity and for a distinct improvement in the abrasion resistance of the plastisols according to the invention after gelation. PA1 The properties can be varied within wide limits by varying the reactive additives without numerous different styrene copolymers having to be separately produced for this purpose. PA1 Low-temperature flexibility is significantly improved.
2. Discussion of Related Art
Although acrylate plastisols of the type known from DE-B-24 54 235 and DE-B-25 29 732 largely meet the technical requirements mentioned at the beginning, the necessary acrylate polymers are also far more expensive than polyvinyl chloride so that, hitherto, the use of such acrylate plastisols has been confined to special applications, for example as spot welding pastes, where PVC plastisols fail completely. Plastisols based on styrene/acrylonitrile copolymers according to EP-A-261 499 are not a satisfactory solution either on account of inadequate abrasion resistance and/or stability in storage.
DE-A-4 139 382 proposes plastisols based on core/shell polymers in which the core of the polymer particles is formed from a diene elastomer while the shell consists of a continuous layer of a methyl methacrylate resin, an acrylonitrile resin or a vinyl chloride polymer.
Although the first two shell materials meet the chlorine-free polymer requirement, polymer particles with a high percentage content of hard shell material are required for storable plastisols. Since these polymer particles are at least partly incompatible with the plasticizer, heterodisperse systems with which it is not possible to obtain optimal performance properties are formed after the gelation process. Although vinyl chloride polymers as a shell material reduce the percentage chlorine content in relation to pure PVC plastisols, they are not chlorine-free and, accordingly, are only an unsatisfactory partial solution to the problem.
EP-A-265 371 describes plastisols of a dispersion of carboxyfunctional fine-particle polymers reacted with polyfunctional basic substances in plasticizers. The polymers used are copolymers of any monomers with polymerizable acids, for example copolymers of vinyl chloride, vinylidene chloride, acrylates, methacrylates, maleates, styrene, methyl styrene, vinyl esters, vinyl ethers, acrylonitrile, olefins or dienes, with acids, such as acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid or fumaric acid. These copolymers are reacted with basic substances, such as basic metal compounds of polyvalent metals, at least bifunctional amine compounds and others. In terms of practical application, these plastisols are unsatisfactory; their mechanical properties (elasticity or breaking elongation) are inadequate. They also show a pronounced tendency towards discoloration and form large bubbles during gelation where polyfunctional amines are added.
According to DE-A-4 034 725, plastisol compositions having excellent performance properties, more particularly high stability in storage, good adhesion to metals and high abrasion resistance, and favorable mechanical properties can be obtained by using as the organic polymer component styrene copolymer powders obtainable by emulsion polymerization which contain
In the emulsion polymerization of these styrene copolymers, polymer particles with a highly uniform average primary particle size of around 0.3 to 1.5 .mu.m can be obtained. In these polymer particles, most of the polar carboxyl groups are externally arranged and, as lipophobic residues, are evidently responsible for the stability of the dispersions of these particles in the plasticizer at room temperature. These copolymers have a molecular weight of the order of 200,000 to 1,000,000. The plastisols produced from them overcome many of the above-described disadvantages of other chlorine-free plastisol systems, more particularly through the possibility of using reactive additives. It has now been found that the basically favorable properties of these plastisols can be significantly improved.